1. Field of the Invention
The present invention is related to optical apparatus and methods for obtaining parameters in an eye examination and, in particular, to optical interferometric apparatus for performing eye examinations.
2. Discussion of Related Art
Refractive surgery to correct refraction error in the human eye has been widely accepted. Several different types of surgical methods have been explored for this purpose. PRK and LASIK are surgical methods that use laser radiation to ablate cornea tissue in order to change the refractive power of the cornea. Phakic IOL surgery implants intra-ocular lenses in either the anterior or posterior chamber of the eye to compensate for refraction error in the eye. To achieve good clinical outcomes from any of these surgeries, physical dimensions of the eye such as cornea thickness, anterior chamber depth, angle-to-angle width, and sucus-to-sucus width, for example, need to be accurately measured pre-operatively. In some cases, post surgery diagnosis, which is important for patient follow up, also requires good measurements of these physical parameters of the eye.
In another common surgery on the eye, cataract surgery has been performed for many years on cataract patients. To achieve an accurate refraction power as a result of the surgery, parameters of the eye such as the axial length, cornea power, anterior chamber depth, and the equatorial plane of the crystalline lens should be accurately measured in order to calculate the power of the intra-ocular lens to be implanted in a Phakic IOL surgery. The true cornea power is especially important when the cataract surgery is performed on a post-Lasik patient.
There are various existing devices that can be used to measure one or two of these parameters, but not all of them in a single apparatus. To acquire all required parameters for surgical preparation, the measurement of various physical parameters of the eye needs to be performed by different instruments. Sometimes, inconsistency in measured results will occur due to discrepancies between the instruments and discrepancies of alignment of the eye with the various instruments. For example, the cornea power can be measured with an Orbscan (by Orbtek, Bausch & Lomb, Rochester, N.Y.) by combination of Pacido ring and slit projection methods. The Pacido ring method uses multiple concentric ring-shaped light sources to illuminate the eye. The cornea is like a mirror and reflects the illumination from the light sources into a CCD camera. The image size and shape of the rings formed by the cornea can then be used to analyze the contour of the cornea. The slit projection method illuminates the cornea with a thin slit of light. The scattering caused by the illuminated cornea tissue can then be imaged in a CCD camera. The cornea thickness and curvature can be calculated from the image of the illuminated cornea.
Due to the slow scan speed, eye motion effects can cause the results of such a test to be highly inaccurate. The anterior chamber depth can be measured either by the slit projection method (IOL Master (by Carl Zeiss, Jena, Germany), Orbscan (by Orbtek, Bausch & Lomb, Rochester, N.Y.), Ultrasound Microscope (UBM by Paradiam Medical, Salt Lake City, Utah), Artemis (by Ultralink, LLC, St. Petersburg, Fla.), B-scan (by Ophthalmic Technologies, Inc., Toronto, Canada), or Optical Coherence Tomography (OCT) (Case Western Reserve University and Cleveland Clinic Foundation). None of these devices is capable of measuring all required parameters in a single compact apparatus.
Because the refractive surgery and cataract surgery can be performed by the same surgeon, it is desired to have a single compact apparatus to measure all of the parameters required by both refractive and cataract surgeries. Therefore, there is a need to provide a single instrument that provides measurements of groups of parameters in order to eliminate inaccuracies due to utilization of several instruments for measurements of these parameters.